Jacques Solomon Hadamard (December 8, 1865 - October 17, 1963 ) was a mathematician best known for his proof of the prime number theorem.

He studied at the École Normale Supérieure. After the Dreyfus affair, which involved him personally, he became politically active and became a staunch supporter of Jewish causes.

He introduced the idea of well-posed problem in the theory of partial differential equations. He also gave his name to the Hadamard inequality on volumes, and the Hadamard matrix, on which the Hadamard transform is based. The Hadamard gate in quantum computing uses this matrix.